This invention relates to a control circuit for a fluid coupling means with a high-gear (direct coupling) clutch capable of being utilized in a torque converter equipped with a high-gear clutch for automatic transmissions.
A conventional control circuit for a fluid coupling equipped with a high-gear clutch is disclosed in the specification of Japanese Patent Kokai Publication No. 60-30864. The control circuit comprises a fluid coupling having a power chamber into which a working fluid is introduced for performing a transmission action between an input element and output element via the fluid, a clutch member arranged for the purpose of connecting and disconnecting the input and output elements as required, a working chamber formed separately of the power chamber, and a piston member forcibly displaced by a pressure differential between the working chamber and power chamber for engaging or disengaging the clutch member; a fluid pressure source provided in order to supply the working fluid to the interior of the power chamber of the fluid coupling; a high-gear clutch control unit which supplies fluid to or discharges the fluid from the working chamber for controlling the connection and disconnection of the clutch; and control means responsive to the supply and discharge of the fluid to and from the working chamber by the high-gear clutch control unit to reduce the pressure internally of the power chamber supplied from the fluid pressure source, thereby enlarging the pressure differential between the working chamber and power chamber.
In other words, in response to operation of the high-gear clutch, the pressure within the torque converter is reduced to enlarge the pressure differential between the high-gear clutch hydraulic pressure and the torque converter hydraulic pressure. The pressure internally of the torque converter is so regulated by a lubricant control valve as to assume a constant value, and lubricant pressure also is regulated simultaneously so as to be constant. Since the lubricating pressure in this case must be regulated to assume a value capable of assuring a prescribed amount of lubricant flow at full throttle, the amount of lubricant that flows at low throttle is greater than the optimum amount, as shown in FIG. 4, the disadvantageous result of which is an increase in oil pump loss.
Similarly, the pressure inside the torque converter at low throttle rises above the optimum pressure. Therefore, in order to obtain a pressure differential of a prescribed magnitude between the high-gear clutch hydraulic pressure and the torque converter pressure, the high-gear clutch hydraulic pressure must be made high. This has an adverse influence upon oil pump efficiency.